Constrained Trajectory Optimization for Planetary Entry via Sequential Convex Programming

Abstract

In this paper, the highly nonlinear planetary-entry optimal control problem is formulated as a sequence of convex problems to facilitate rapid solution. The nonconvex control constraint is avoided by introducing a new state variable to the original three-dimensional equations of motion. The nonconvex objective function and path constraints are convexified by first-order Taylor-series expansions, and the nonconvex terms in the dynamics are approximated by successive linearizations. A successive solution procedure is developed to find an approximated solution to the original problem, and its convergence is discussed. In each iteration, a convex optimization problem is solved by the state-of-the-art interior-point method with deterministic convergence properties. Finally, the proposed method is verified and compared to a general-purpose optimal control solver by numerical solutions of minimum terminal-velocity and minimum heat-load entry problems. The sequential method converges to accurate solutions with faster speed than the general-purpose solver using MATLAB on a desktop computer with a 64-bit operating system and an Intel Xeon E3-1225 V2 3.2 GHz processor, which demonstrates its potential real-time application for computational guidance.